The present invention relates to a method for determining material characteristics in a borehole, being more particularly directed to a method for determining the presence and properties of materials behind or outside casing by acoustic wave measurements.
For many years acoustic logging has been used in cased wells for the purpose of determining the quality of the casing's cementation. The principle consisted of interpreting the amplitude of an acoustic signal received at some spacing from a transmitter in the borehole fluid. Improvements on this technique included using an array of transducers and thus deriving the attenuation rate of the signal along a defined length of casing. This attenuation rate is primarily a function of the mechanical properties of the cement that is bonded to the casing over that length.
The present invention is an improvement over previous acoustic logging with the two primary innovations in the present invention being concerned with:
1. The amplitude of the received acoustic signal being separated into two components: (i) a part due to the coupling of the acoustic energy between the wave in the borehole fluid and the wave propagating axially along the casing, and (ii) a part due to the attenuation of the wave propagating along the casing.
2. The part due to coupling being not only a function of the casing and of the borehole fluid, but also being useful to determine mechanical properties of the material outside the casing independently of its bond to the casing. From these properties, micro- and macro- annulus can be differentiated from free pipe or well bonded pipe.
In background, the "casing wave" measured in classical acoustical cement bond logging is a pressure wave reradiated back into the borehole fluid and there to a receiving acoustic transducer on the well logging instrument by an elastic wave passing axially along the steel casing. This elastic wave propagates as a Lamb mode of the thin steel cylinder. It is itself typically excited in the casing by a pressure wave emanating from a transmitting acoustic transducer in the well logging instrument. Prior industry practice has been to measure either amplitude of the casing wave, or to determine its spatial attenuation rate by taking amplitude measurements at different transmitter-receiver spacings.
Cement bond logs have traditionally been made by measuring the amplitude of the 1st significant acoustic arrival, generally referred to as the "E1" peak of the transient casing wave. This and other wavefronts are sensed by a receiving transducer separated from an impulsive acoustic transmitter by a preselected distance, such as 3.0 ft., along the axis of the casing. In some measurement techniques peaks other than E1 and/or spacings other than 3.0 ft. are chosen. Additionally, in some bond log data presentations, amplitude reductions relative to the amplitude measured at a single receiver in the "free", water backed, casing condition are converted to a pseudo-attenuation rate measurement.
An advantage of the present invention is the recognition that the amplitude reduction between transmitter and receiver depends not only on attenuation losses due to axial propagation along the casing, but also on the efficiency of acoustical coupling into and out of the casing wave. For single spacing measurement systems, for example one transmitter and one receiver, it is not possible to separate effectively these independent contributions to amplitude decrement. Amplitude reduction from all sources has often, mistakenly, been treated together and considered to be due to propagation along the casing. When additional casing wave amplitude controlling factors have been recognized they have not included the mechanical properties of the material outside and adjacent to the casing. Rather, properties of the materials outside the casing have been considered to control only the magnitude of the propagation attenuation rate, referred to also as the attenuation coefficient.
Recently, multireceiver cement bond tools have been introduced which are capable of measuring casing wave peak heights at more than a single transmitter-receiver spacing. One such tool is the Cement Bond Tool (CBT) produced by the instant assignee. Multispacing data provides a more complete description of the casing wave amplitude decay versus distance, and provides information necessary to determine the spatial attenuation rate itself. In the U.S. patent application Ser. No. 394,395, now Pat. No. 4,757,479, issued July 12, 1988 entitled "Method and Apparatus for Cement Bond Logging", filed July 1, 1982 and assigned commonly, incorporated by reference, a means of computing the spatial attenuation rate is disclosed along with appropriate apparatus in which factors affecting peak acoustic amplitudes other than the attenuation rate are cancelled out. By this means a compensated attenuation rate measurement is made which is independent of differences or changes in transmitter power outputs and receiver sensitivities and also is relatively independent of environmental factors such as borehole fluid properties.
Measurement techniques using multiple spacings are thus capable both of determining true attenuation rates and of separating them from other variations of amplitude, which are not associated with axial propagation along the casing. No successful attempt has previously been made, however, to exploit these capabilities and make use of that part of the amplitude variation which is not directly due to attenuation along the casing. Factors which will affect the absolute amplitude of the received casing wave signal have been reported as including only "environmental factors" such as borehole fluid properties, and the temperature or pressure characteristics of the measuring apparatus, not the properties of material outside the casing.
A particularly important shortcoming of previous cement evaluation logging systems which measure and rely only on the attenuation rate measurement is that this measurement is strongly affected by the microannulus which often appears between casing and the solid cement column after the cement has set up or solidified. These microseparations may occur either within the cement sheath itself, or at the interfaces of the cement column with the steel casing or formation. They may result from several factors including expansion and/or contraction of the casing due to either temperature or pressure cycles or borehole fluid changes or shock and vibration occurring during the well completion process, or to shrinkage of the cement itself. Often these microseparations are small enough that the permeability of the annulus is not significantly modified, and the hydraulic seal offered by the cement column not impaired. Microannuli do, however, severely reduce the acoustic attenuation rate measured by tools which determine the attenuation rate, and they cause the received amplitude to increase for tools which measure the peak amplitude only. Those measurements are thus not reliable measurements for use in estimating the hydraulic seal.
In the practice of the present invention, amplitude change between transmitter and receiver is separated into a change dependent on coupling into and out of the casing wave, as well as a change related to attenuation along the casing. Coupling is therefore an important amplitude controlling mechanism which depends on the properties of the material outside of the casing, but in functionally different ways than the attenuation. In particular, it has been found that the mechanical bonding between the casing and the cement sheath is not a dominant factor in the coupling.
Coupling magnitude additionally appears to be controlled largely by the compressional impedance of the material backing the casing and to a lesser degree by that material's Poisson ratio. Attenuation rate, on the other hand, is controlled mainly by the shear wave speed of the material which is bonded to the casing and thus is sensitive to the Poisson ratio as well as the magnitude of the compressional impedance. As another functional difference, coupling is controlled by the distribution of solid material in the annulus, that is, the percentage of casing surface area backed by cement within the investigation region of the coupling measurement, while attenuation is controlled by the percentage of surface area bonded to the cement.
The signals corresponding to coupling and attenuation rate thus respond differently to mechanical properties and geometrical configurations of the material behind the casing. Measurement of both coupling and attenuation allows determination of the presence of microannulus and estimation of average mechanical properties of material behind casing at each depth.